Shake the phone to start. In EC a transformer with coupling coefficient greater than unity (which is impossible in real life) can be used to create a negative inductance, like the one in this circuit.
A negative inductor is a source of energy, therefore, usually, if you connect it to an element with positive parameters (a resistor, for instance) the circuit becomes unstable. However, when a negative L is connected to a negative C, like in this impedance inverter circuit, the whole circuit is stable and behaves similarly to a normal LC tank.
Try also setting the coupling coefficient in the transformer below unity: you will see that the circuit turns into a generator, i.e., becomes unstable again.
